As described in U.S. Pat. Nos. 4,274,007, 4,445,042, 4,447,733, and 4,428,454 a radiation-shielding storage and transport container for radioactive wastes, for instance spent fuel rods, comprises an upright one-piece cast-iron vessel with an upwardly open mouth formed with seat-forming annular steps. A massive inner cover or plug is secured to the lowest and innermost step, typically by bolting, and at least one lighter-duty outer cover is welded into place in the outer step. The atmosphere in the space between the inner and outer covers can be sampled to see if the container is leaking past the plug.
Accurately forming the cover-receiving seats and securing the covers in place is fairly difficult mainly because cast iron is difficult to turn, bore, and otherwise machine. Since these seats must be made to high tolerances, it is impossible to produce them by casting, so that one must always resort to the difficult procedures entailed in the material-removing machining of cast iron.